1. Field of the Invention
This invention relates to an ultrasonic cleaning apparatus, more specifically, to an apparatus that utilizes ultrasonic waves to clean substrates, for example, semiconductor wafers or LCD glass substrates.
2. Description of the Related Art
In the processing of semiconductor devices or LCDs, the wet-method for cleaning semiconductor wafers or LCD glass substrates by immersing them into a tank containing a chemical liquid or rinse liquids, etc., is widely utilized.
One example of a cleaning apparatus that implements the wet-cleaning method is an ultrasonic cleaning apparatus. Conventional ultrasonic cleaning apparatuses are primarily comprised of a quartz cleaning tank that holds cleaning liquids such as chemical fluids in which the semiconductor wafer is immersed, an intermediate tank that that holds vibration-propagating liquid such as pure-water in which the bottom portion of the cleaning tank is immersed, and an ultrasonic wave generating unit comprised of a stainless steel vibration transfer plate, on which ultrasonic transducers are affixed, arranged at a bottom portion of the intermediate tank.
With ultrasonic cleaning apparatuses like that mentioned above, semiconductor wafers held by a wafer guide are immersed in the cleaning tank that contains chemical fluids, the ultrasonic transducers are excited by the application of an appropriate high-frequency voltage, thereby generating ultrasonic wave. The ultrasonic wave propagates through the pure water in the cleaning tank, and removes particles etc. adhered to the wafers.
The acoustic pressure of the ultrasonic wave and processing time determines the efficiency of particle removal. Particles can be removed in a shorter amount of time if the acoustic pressure of the ultrasonic wave is high. Even if the acoustic pressure is low, particles can be removed by lengthening the processing time. Conventional ultrasonic cleaning apparatuses have been geared toward faster, high acoustic pressure processing, or slow, low acoustic pressure processing. With the former, damage to the wafer may result, and with the latter, the problem of low throughput is a concern.
Chemical liquids used with ultrasonic cleaning apparatuses include a mixture of ammonia solution and hydrogen peroxide solution called “SC1” (NH4OH/H2O2/H2O), a mixture of hydrochloric acid solution and hydrogen peroxide solution called “SC2” (HCl/H2O2/H2O), and dilute hydrofluoric acid (DHF ,etc., and these are used according to the objective of the cleaning process. When these chemical liquids are used, dissolving of the quartz cleaning tank may occur, resulting in generation of metallic contamination. Also, with the dissolving of the cleaning tank, the through rate of sonic power may be affected, causing the possibility that cleansing efficiency may be worsened.